The present invention relates to polycarbonate-polyester compositions. More particularly, the present invention relates to improved polyester-polycarbonate compositions stabilized against ester-carbonate interchange.
Polyester-polycarbonate compositions are widely used in industry. However, a disadvantage of these compositions is their tendency to undergo ester-carbonate interchange, wherein ester linkages in both the polycarbonate and the polyester are believed to be broken and replaced by alkylene carbonate and aryl carboxylate bonds. The result is degradation of the physical properties of the polymers due to hybridization of the molecular linkages therein. This in turn leads to variability in the final fabricated article.
It is believed that the ester-carbonate interchange in polyester-polycarbonate compositions is promoted by metallic catalyst residues present in the polyester. These residues are left over from the polymerization reaction forming the polyester, wherein certain metal compounds are used as polymerization catalysts. It appears, however, that these metal compounds also catalyze the transesterification reaction between the polycarbonate and the polyester.
It would be desirable to provide a compound which deactivates the metallic catalyst residues present in the polycarbonate/polyester compositions. The resulting polyester-polycarbonate compositions would be improved in that they would have a decreased tendency to undergo ester-carbonate interchange and therefore would be stable against such interchange.
It is known in the art that certain phosphorous-containing inorganic compounds are useful in deactivating metallic catalyst residues. Reference is made, for example, to U.S. Pat. Nos. 4,532,290 (Jaquiss et al.) and 4,401,804 (Wooten et al.).
Copending, commonly assigned U.S. Patent Application Docket Number 8CV-5282, filed Jun. 2, 1992, to Douglas G. Hamilton (Serial Number not yet assigned) discloses the use of silyl phosphates to deactive metallic catalyst residues in polyester-polycarbonate compositions.
The present invention is based on the discovery that certain organoorthosilicates compounds will effectively deactivate metallic catalyst residues in a polycarbonate-polyester composition.